MOTOR TRUCK SERVICE MANUAL BRAKES-PARKING
release) in the event of air failure on the road, for towing,
or relining brakes, the spring brake can be released by
removing the breather cap and backing off
(counterclockwise) the release bolt, approximately 13/4",
Before releasing spring brakes, be sure vehicle is
properly blocked so it cannot move when the brakes are
released. For towing, make sure vehicle is connected or
secured to tow vehicle before releasing spring brakes.
The following four sectional views of the parking
brake show the various stages of operation:
Fig. 5 Parking Brake in Released Position and Service
Brake in Applied Position (Air Pressure Applied to Both).
Fig. 5 illustrates normal operation of both spring
brake and service brake. The spring brake does not
affect normal application of the service brakes; each unit
Fig. 4 Parking Brake and Service Brake in Released
Position (Air Pressure Applied to Parking Brake Spring).
Fig. 4 illustrates the spring brake with the spring in a
loaded position. The force of air pressure against the
piston keeps the powerful spring compressed. Notice
position of release bolt (in a down position). The service
brake is shown in a released position.
DO NOT attempt to perform any
disassembly whatsoever of the parking brake (either on
the vehicle or with the brake assembly removed) with the
air pressure applied as shown in Fig.4. The parking
brake spring while in the compressed position, Fig. 4, is
exerting a force against the head of the chamber of
approximately 1800 lb. on a 20" or 24" chamber or 2300
lb. on a 30" chamber.
Fig. 6 Parking Brake in the Applied Position with Service
Brake Applied through Parking Brake Push Rod (Loss of
Fig. 6 illustrates the loss of air pressure. The loss of
air pressure below safe